Measuring multiple frequency responses of a subterranean formation with a logging tool such as a high definition induction logging tool (HDIL) or a three-dimensional (3D) explorer (3DEX), is very common for formation evaluations. However, only a single frequency or perhaps two frequencies are customarily used for logging while drilling (LWD) tools. For example, the OnTrak® LWD system commercially offered by the Assignee of the present invention employs two frequencies to measure formation resistivity while drilling. The two frequencies (400 kHz and 2 Mhz) offer different depths of investigation so that a resistivity profile can be better resolved.
On the other hand, for certain measurements, such as azimuthal propagation resistivity, measurement at a lower frequency, for example around 100 kHz, can be beneficial.
As is known in the art, for some tools, a broadband antenna without tuning may be used, or the antenna may be tuned separately to two frequencies using known techniques that do not involve the use of switches or relays. It has also been shown that an antenna can be tuned to more than two frequencies through the use of switches or relays. An example of the latter is described in U.S. Pat. No. 7,038,455 to Beste et al., entitled “Electromagnetic Wave Resistivity Tool.” The Beste et al. '455 patent is hereby incorporated by reference herein in its entirety.
In typical prior art arrangements, such as that described in the Beste et al. '455 patent, for each antenna on the tool, there is an associated junction box formed in the wall of the tool. The junction box serves to contain the tuning circuitry for the antenna, including any switches or relays required for multiple-frequency tuning. The junction box for a receiving antenna typically also contains a pick-up coil used to detect current in the antenna. Such an arrangement is described in further detail in U.S. Pat. No. 5,530,358 to Wisler et al., entitled “Method and Apparatus for Measurement While Drilling Utilizing Improved Antennas.” The Wisler et al. '358 patent is commonly assigned to the Assignee of the present invention and is hereby incorporated by reference herein in its entirety.
Since junction boxes are machined into the sidewall of the tubular tool body, it is necessary to enclose or seal them, such as with an epoxy or the like, in order to protect the circuitry within the junction box from the harsh conditions to which tools are exposed during utilization. Even with such protective materials, there is a risk that elements such as switches or relays contained within a junction box may not be durable enough to withstand the harsh conditions of a drilling operation. Incorporating such delicate components can thus result in a shorter operational life for the tool.
As noted in the Wisler et al. '358 patent, a further concern is that the tubular tool is weakened wherever junction boxes are formed in the sidewall. Machining of junction boxes into a tubular element's sidewall jeopardizes the structural integrity and load-bearing ability of the tool, to an extent that may not be acceptable in particularly difficult drilling operations.